The software is open source and there are many, many copies of it throughout the bitcoin network. I think it would take universal agreement (impossible) across all bitcoin holders to make a change -- and why would they.
On the other hand, there's nothing to prevent people from firing up bitcoin clones based on the same algorithms, or bitcoin lookalikes based on slightly different algorithms. As a matter of fact there are many virtual currencies out in the wild. For a while (I don't know if it's still the case), companies were starting up new virtual currencies as a way to fund venture startups.
OT.....how did the first bitcoin get started?